Multilayer optical disk

ABSTRACT

A multilayer optical disk having an information storage layer which can as well as be reproduced by a general purpose reproducing apparatus, for example, a compact disk player, and from which information can be read from other information storage layers by using an exclusive reproducing apparatus. The method of manufacturing a multilayer disk comprises the steps of forming a first substrate having a first information storage area enabling reproduction of information therein with a first light beam having a wavelength of 770 nm to 830 nm; forming a second substrate having a second information storage area enabling reproduction of information therein with a second light beam having a wavelength of 615-655 nm but which is relatively transparent with respect to said first light beam; and bonding said first substrate to said second substrate together without said first and second said information areas facing each other.

RELATED APPLICATION DATA

[0001] This application claims priority to and is a continuation of U.S.patent application Ser. No. 09/086,277 filed May 28, 1998 (allowed),which in turn claims priority to and is a continuation of Ser. No.09/023,961 filed Feb. 13, 1998, which in turn claims priority to and isa continuation of Ser. No. 08/629,564 filed Apr. 9, 1996, now U.S. Pat.No. 5,766,717 which issued on Jun. 16, 1998. This application alsoclaims priority to Japanese patent applications Nos. P07-085369 filedApr. 11, 1995 and P07-094068 filed Apr. 19, 1995. The disclosures of allof the foregoing applications are incorporated herein by reference tothe extent permitted by law.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a multilayer optical disk havinga multilayer structure consisting of a plurality of layers for storinginformation.

[0003] In recent years, the popularity of so-called multimedia resultsin a necessity of handling large-capacity information such as a digitalmovie data. Therefore, a necessity arises in that large-capacityinformation of the foregoing type must be stored and randomly accessedso as to be reproduced, if need arises.

[0004] An optical disk is a storage medium permitting random access,exhibiting a large capacity and capable of being ejected (i.e., beingremovable) from a playback or reproducing apparatus. Therefore, opticaldisks have been used in a variety of fields and large quantities. Inorder to enlarge the capacity as described above, one side of theoptical disk must be capable of handling large-capacity information.

[0005] Under the foregoing circumstance, the capacity of the opticaldisk has been enlarged by providing for a multiplicity of layers forstoring information that are laminated in the direction of the thicknessof the optical disk.

[0006] In such a multilayer optical disk, a multilayer structure isformed on one side thereof and consists of information layers forstoring information, wherein the focal points are made so as to bedifferent from one another among the multiple layers so that informationis read while maintaining a random access characteristic and, therefore,large-capacity information is handled by a large capacity.

[0007] Reported and representative multilayer optical disks will now bedescribed.

[0008] 1. A concept for reproducing the optical disk by making thepositions of the focal points to be different from one another isreported, for example, U.S. Pat. No. 3,946,367.

[0009] 2. A method using a multilayer disk having an information layerformed by laminating a plurality of layers on one side of the disk sothat information is read by means of transmitted light or reflectedlight is reported, for example, U.S. Pat. No. 4,219,704.

[0010] 3. A system for reproducing information from a multilayer opticaldisk, the system comprising an optical system having an aberrationcorrection function (refer to U.S. Pat. No. 5,202,875).

[0011] In order to reproduce data in a conventional multilayer opticaldisk of the foregoing type, an exclusive apparatus for reproducing thedata in the optical disk is required. For example, there arises arequirement for a reproducing optical system capable of independentlyreproducing a reproduction signal from the first information storagelayer or the second information storage layer without mixing of data. Asan alternative to this, a signal processing system is required which iscapable of distinguishing reproduction signals from the firstinformation storage layer and the second information storage layer fromeach other so as to extract information without confusion.

[0012] Therefore, a reproducing apparatus, such as a compact disk playerfor reproducing a so-called compact disk (CD) which has been used widelyas a read-only optical disk, cannot be used as it is.

SUMMARY OF THE INVENTION

[0013] Accordingly, an object of the present invention is to provide anovel multilayer optical disk having an information storage layer, whosedata on at least one information storage layer can be reproduced by ageneral purpose reproducing apparatus, such as a compact disk player,and whose data on another information storage layer can be reproduced byan exclusive reproducing apparatus.

[0014] To that end, in an embodiment, a method of manufacturing amultilayer optical disk, comprising the steps of:

[0015] forming a first substrate having a first information storage areaenabling reproduction of information therein with a first light beamhaving a wavelength of 770 nm to 830 nm; forming a second substratehaving a second information storage area enabling reproduction ofinformation therein with a second light beam having a wavelength of615-655 nm but which is relatively transparent with respect to saidfirst light beam; and bonding said first substrate to said secondsubstrate together without said first and second said information areasfacing each other.

[0016] In an embodiment, the invention provides that the first substrateis formed using an injection molding process.

[0017] In an embodiment, the invention provides that the secondsubstrate is formed using an injection molding process.

[0018] In an embodiment, the invention provides that the firstinformation storage area comprises an Al film layer.

[0019] In an embodiment, the invention provides that the secondinformation storage area comprises one or more layers of a Si—N film.

[0020] In an embodiment, the invention provides that the firstinformation storage area has a reflection factor of about 70% or morewith respect to the first light beam.

[0021] In an embodiment, the invention provides that the secondinformation storage area has a reflection factor of 20% or more withrespect to the second light beam.

[0022] In an embodiment, the invention provides that when the secondsubstrate is formed, projections corresponding to recording pits areformed thereon.

[0023] In an embodiment, the invention provides that the first substrateis formed with projections thereby corresponding to recording pitsformatted on a compact disk recording format.

[0024] These and other features of the invention are discussed ingreater detail below in the following detailed description of thepresently preferred embodiments with reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a schematic view showing the basic structure of amultilayer optical disk according to the present invention;

[0026]FIG. 2 is a schematic cross sectional view showing essentialportions of an example of the structure of the multilayer optical diskaccording to the present invention;

[0027]FIG. 3 is a graph showing the spectroscopic characteristic of thereflection factor of the multilayer optical disk shown in FIG. 2;

[0028]FIG. 4 is a graph showing the spectroscopic characteristic of thereflection factor of a multilayer optical disk having a secondinformation-storage layer formed into a triple-layer structure;

[0029]FIG. 5 is a graph showing the spectroscopic characteristic of thereflection factor of a multilayer optical disk having a secondinformation-storage layer formed into a semiconductor layer materialfilm;

[0030]FIG. 6 is a schematic cross sectional view showing essentialportions of an example of a multilayer optical disk having a structurecomprising two substrates bonded to each other;

[0031]FIG. 7 is a schematic cross sectional view showing essentialportions of another example of the multilayer optical disk having astructure comprising two substrates bonded to each other;

[0032]FIG. 8 is a schematic cross sectional view showing essentialportions of a multilayer optical disk having a first information-storagelayer formed adjacent to the substrate and formed by a triple-layerstructure film;

[0033]FIG. 9 is a graph showing the spectroscopic characteristic of thereflection factor of the multilayer optical disk shown in FIG. 7; and

[0034]FIG. 10 is a graph showing the spectroscopic characteristic of thereflection factor of a multilayer optical disk having two informationstorage layers which are Si films.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Preferred embodiments of the present invention will now bedescribed in detail with reference to the drawings.

[0036] First Embodiment

[0037] As shown in FIG. 2, a multilayer optical disk according to thisembodiment has a structure such that a second information-storage layer13, a spacer layer or substrate 14 and a first information-storage layer12 are, in this sequential order, formed on a substrate 11.

[0038] The substrate 11 is made of, for example, polycarbonate or glass.If the substrate 11 is made of polycarbonate, a so-called 2P method isemployed in such a manner that a projection and pit pattern, such asrecording pits, is formed as information to be exclusively reproduced.In this embodiment, a polycarbonate substrate having a thickness of 1.2mm and made by injection molding is employed.

[0039] The second information-storage layer 13 is formed along theprojection and pit pattern of the substrate 11 to reflect a portion oflight beams made incident upon the substrate 11 and permit a portion ofthe light beam to transmit therethrough. Thus, the secondinformation-storage layer 13 serves as a reflecting film for readinginformation in the form of the projection and pit pattern to beexclusively reproduced.

[0040] The spacer layer or substrate 14 is formed to optically separatethe second information-storage layer 13 and the firstinformation-storage layer 12 from each other. Therefore, the spacerlayer 14 must have a certain thickness. Specifically, it is preferablethat the spacer layer 14 be 30 mm or thicker. If the spacer layer 14 istoo thin, light reflected by the first information-storage layer 12 andthat reflected by the second information-storage layer 13 cannotsufficiently be separated from each other and, therefore, accuratedetection cannot easily be performed. If the spacer layer 14 is toothick, spherical aberration and the like take place. Therefore, thethickness must be determined appropriately to prevent the foregoingproblems.

[0041] The first information-storage layer 12 formed on the secondinformation-storage layer 13 through the spacer layer 14 is made of amaterial having a high reflection factor. The reflection factor of thefirst information-storage layer 12 is 70% or higher with respect to alight beam having a wavelength of 770 nm to 830 nm.

[0042] To manufacture the foregoing multilayer optical disk, thesubstrate 11 is prepared initially, the substrate 11 having theprojection and pit pattern, such as pits, formed to correspond toinformation signals to be read from the second information-storage layer13. The substrate 11 may be made of either glass or plastic such aspolycarbonate plastic. In the case where the glass substrate isemployed, a so-called 2P (Photo Polymerization) method or the like isperformed with which photosetting resin is enclosed between the glasssubstrate and a disk stamper, and then the photosetting resin isirradiated with light from a position on the outside of the glasssubstrate so that the projection and pit pattern is formed. Also in thecase of the plastic substrate, the projection and pit pattern may beformed by the 2P method. However, an injection molding method using astamper is generally employed to form the projection and pit pattern.

[0043] The second information-storage layer 13 is, on the substrate 11,formed by a vacuum evaporation method or a sputtering method.

[0044] Then, the spacer layer 14 is formed on the secondinformation-storage layer 13. Since the spacer layer 14 must have acertain thickness, for example, a thickness of 30 nm or more, the spacerlayer 14 is formed by applying ultraviolet-curing resin or the like by aspin coating method. The spacer layer 14 may be formed into a laminatedshape by plural times of the laminating processes in each of which alayer having a thickness of μm to 10 μm is formed. The spacer layer 14may be formed by bonding a transparent sheet.

[0045] Also the spacer layer 14 must have pits or the like formed tocorrespond to the information signals to be recorded on the firstinformation-storage layer 12. The foregoing pits can be formed by the 2Pmethod similar to the foregoing projection and pit pattern.

[0046] After the spacer layer 14 has been formed, the first informationstorage layer 12 is formed on the spacer layer 14. Then, if necessary, aprotective film 15 is formed by ultraviolet-curing resin or the like.

[0047] In this embodiment, the second information-storage layer 13 onthe substrate 11 is in the form of a five-layer structure filmconsisting of Si₃N₄/SiO₂/Si₃N₄/SiO₂/Si₃N₄ layers. The five layersrespectively have thicknesses of 64 nm/90 nm/64 nm/90 nm/64 nm.

[0048] The refractive index n of Si₃N₄ is 2.0 and the extinctioncoefficient k of the same is 0, while the refractive index n of SiO₂ is1.5 and the extinction coefficient k of the same is 0. As a matter ofcourse, materials having similar optical characteristics may be employedto obtain a similar characteristic.

[0049] The first information—storage layer 12 is an Al film having athickness of 100 nm.

[0050] In the thus-structured multilayer optical disk, the reflectionfactor R2 of the second information-storage layer 13 (the five-layerstructure film) has a spectroscopic characteristic as shown in FIG. 3such that the reflection factor R2 is 34% with respect to light having awavelength of 635 nm; and the reflection factor R2 is substantially zerowith respect to light having the wavelength of 780 nm. That is, thesecond information-storage layer 13 has somewhat refractivity withrespect to light having the wavelength of 635 nm and permits lighthaving the wavelength of 780 nm to substantially transmit therethrough.

[0051] On the other hand, the first information-storage layer 12 (the Alfilm) has a reflection factor of 80% or higher with respect to both oflight beams which have penetrated the second information-storage layer13. Therefore, the reflection factor R1 of the first information-storagelayer 12 with respect to the reproducing light beam is about 84% withrespect to light having the wavelength of 780 nm which substantiallypenetrates the second information-storage layer 13 and about 38% withrespect to light having the wavelength of 635 nm which has somewhatpenetrated the second information-storage layer 13.

[0052] With the multilayer optical disk having the foregoing structure,use of the reproducing light beam having the wavelength of 780 nmenables reflected light to be obtained from only the firstinformation-storage layer 12 so that information signals recorded on thefirst information-storage layer 12 are read. Since the reproducing lightbeam having the wavelength of 780 nm is used in the compact disk playerand the like, the information signals recorded on the firstinformation-storage layer 12 can as well as be reproduced by theforegoing general-purpose reproducing apparatus. Since both of the firstinformation-storage layer 12 and the second information-storage layer 13have the reflection factor of about 30% with respect to the reproducinglight beam having the wavelength of 635 nm, information signals can beobtained from the two layers 12 and 13. Thus, the positions of the focalpoints of the reproducing light beams are made to be different from eachother so that information is read from both of the firstinformation-storage layer 12 and the second information-storage layer13.

[0053] Second Embodiment

[0054] A multilayer optical disk was manufactured which comprised asecond information-storage layer 13 in the form of a triple—layerstructure film and other layers respectively having structures similarto those of the first embodiment. Note that the secondinformation-storage layer 13 was in the form of a triple-layer structurefilm, the triple layers respectively being made of Si_(x)N_((1-x))(n=2.8 and k=0)/Si₃N₄ (n=2 and k=0)/Si_(x)N_((1-x)) (n=2.8 and k=0). Thethicknesses of the respective layers were 40 nm/73 nm/40 nm.

[0055] Also in this embodiment, the reflection factor Ri of the firstinformation-storage layer 12 and the reflection factor R2 of the secondinformation-storage layer 13 have the spectroscopic characteristics asshown in FIG. 4. With the reproducing light beam having the wavelengthof 780 nm, only the information signals recorded on the firstinformation-storage layer 12 can be read. With the reproducing lightbeam having the wavelength of 635 nm, information signals can be readfrom both of the first information-storage layer 12 and the secondinformation-storage layer 13.

[0056] Third Embodiment

[0057] In this embodiment, the second information-storage layer 13 ismade of Al_(x)Ga_((1-x))As which is a semiconductor laser material andwhich has a thickness of 120 nm.

[0058] The first information-storage layer 12 is an Al film having athickness of 100 nm. The other structures are the same as thoseaccording to the first embodiment.

[0059] In the multilayer optical disk structured as described above, thereflection factor R2 of the second information-storage layer 13 has aspectroscopic characteristic as shown in FIG. 5 such that the reflectionfactor is about 32% with respect to light having the wavelength of 635nm and substantially zero with respect to light having the wavelength of780 nm. That is, the second information-storage layer 13 has somerefractivity with respect to light having the wavelength of 635 nm andpermits light having the wavelength of 780 nm to substantially transmittherethrough.

[0060] On the other hand, the first information-storage layer 12 (the Alfilm) has a reflection factor of 80% or higher with respect to both ofthe light beams which have penetrated the second information-storagelayer 13. Therefore, the reflection factor R1 of the firstinformation-storage layer 12 with respect to light having the wavelengthof 780 nm which substantially penetrates the second information-storagelayer 13 is about 84% and the reflection factor R1 is about 36% withrespect to light having the wavelength of 635 nm which has substantiallypenetrated the second information-storage layer 13.

[0061] With the multilayer optical disk having the foregoing structure,use of the reproducing light beam having the wavelength of 780 nmenables reflected light to be obtained from only the firstinformation-storage layer 12 so that information signals are read fromonly the first information-storage layer 12. On the other hand, sinceboth of the first information-storage layer 12 and the secondinformation-storage layer 13 have the reflection factor of 30% or higherwith respect to the reproducing light beam having the wavelength of 635nm, information signals can be obtained from both of the layers 12 and13. By making the positions of the focal points of the reproducing lightbeams to be different from each other, information can be read from thetwo layers 12 and 13.

[0062] Fourth Embodiment

[0063] As shown in FIG. 6, this embodiment has a structure such that asecond information-storage layer 23 is formed on a substrate 21 having athickness of 0.6 mm; and a first information-storage layer 22 is formedon a second substrate 31 having a thickness of 0.6 mm, the twosubstrates 21 and 31 being then bonded to each other through a bondingadhesive layer 24 having light transmission properties. Thus, amultilayer optical disk according to this embodiment is constituted.

[0064] Each of the substrates 21 and 31 is a polycarbonate substrateobtained by injection molding or a glass substrate manufactured by the2P method. The structures of the information storage layers 22 and 23are similar to those according to the first embodiment. A protectivefilm 25 is formed on the first information-storage layer 22.

[0065] Also with the multilayer optical disk having the foregoingstructure, similar to the first embodiment, reflected light was obtainedfrom only the first information-storage layer 22 with respect to thereproducing light beam having the wavelength of 780 nm. With respect tothe reproducing light beam having the wavelength of 635 nm, reflectedlight was obtained from the first information-storage layer 22 and thesecond information-storage layer 23.

[0066] This embodiment may have another structure formed such that thefirst substrate and the second substrate are bonded to each other withthe bonding adhesive layer 24 in such a manner that the firstinformation-storage layer 22 and the second information-storage layer 23face each other.

[0067] Fifth Embodiment

[0068] As shown in FIG. 7, this embodiment has a structure arranged suchthat a second information-storage layer 43 is formed on a firstsubstrate 41 having a thickness of 1.2 mm, a first information-storagelayer 42 is formed on a second substrate 51 having a thickness of 0.6mm, and the first and second substrates 41 and 51 are, by a bondingadhesive layer 44, bonded to each other in such a manner that theinformation-storage layers 42 and 43 face each other.

[0069] Each of the substrates 41 and 51 is a polycarbonate substrateformed by injection molding. The structure of each of the informationstorage layers 42 and 43 is similar to that of the first embodiment.

[0070] Also with the multilayer optical disk having the foregoingstructure, similar to the first embodiment, reflected light was obtainedfrom only the first information-storage layer 42 with respect to thereproducing light beam having the wavelength of 780 nm. With respect tothe reproducing light beam having the wavelength of 635 nm, reflectedlight was obtained from both of the first information-storage layer 42and second information-storage layer 43.

[0071] Since this embodiment has the structure such that the firstinformation-storage layer 42 and the second information-storage layer 43are formed on the corresponding substrates 41 and 5J, which are thepolycarbonate substrates obtained by injection molding, a technique,such as the 2P method, is not required. Therefore, the multilayeroptical disk according to this embodiment can significantly easily bemanufactured.

[0072] Sixth Embodiment

[0073] As shown in FIG. 8, this embodiment has the structure arrangedsuch that a first information-storage layer 62 was formed adjacent to asubstrate 61, and then a second information-storage layer 63 was formedon the first information-storage layer 62 through a spacer layer ofsubstrate 64.

[0074] The first information-storage layer 62 consists of three layerswhich respectively are made of Si_(x)N(1−x)(n=3.5 and k=O)/SiO₂(n=1.5and k=0)/Si_(x)N_((1-x)) (n=3.5 and k=0) respectively having thicknessesof 45 nm/195 nm/45 nm.

[0075] The second information-storage layer was an Al film.

[0076] Also the multilayer optical disk having the foregoing structureenabled a spectroscopic characteristic of the reflection factor as shownin FIG. 9 to be obtained. With respect to the reproducing light beamhaving the wavelength of 780 nm, reflected light was obtained from onlythe first information-storage layer 62 as indicated by symbol R1 shownin FIG. 9. With respect to the reproducing light beam having thewavelength of 635 nm, the first information-storage layer had the lighttransmission properties so that reflected light was obtained from thesecond information-storage layer 63 as indicated by symbol R2 shown inFIG. 9.

COMPARATIVE EXAMPLE

[0077] A multilayer optical disk was manufactured which comprised a Sifilms each having a thickness of 14 nm, the Si films being formed intothe first and second information storage layers. Then, the spectroscopiccharacteristic of the reflection factor of each Si film was measured.

[0078] As a result, the reflection factor of each Si layer had nodependency upon the wavelength as shown in FIG. 10. Since the reflectionfactor is, in particular, low with respect to light having thewavelength of 780 nm, the compatibility with the compact disk and thelike cannot be maintained.

[0079] Note that the present invention has the structure such that theinformation storage layer corresponding to the light beam having thewavelength of 780 nm is formed in a CD format that can be reproduced bya compact disk player. On the other hand, the information storage layercorresponding to only the light beam having the wavelength of 635 mn isrecorded at a density higher than, for example, that of the CD format.

[0080] Although the invention has been described in its preferred formwith a certain degree of particularity, it is understood that thepresent disclosure of the preferred form can be changed in the detailsof construction and in the combination and arrangement of parts withoutdeparting from the spirit and the scope of the invention as hereinafterclaimed.

What is claimed is:
 1. An optical disc comprising: a first informationstorage layer which is relatively reflective with respect to a firstlaser beam but is relatively transmissive with respect to a second laserbeam, the wavelength of said second laser beam being shorter than thewavelength of said first laser beam, the information stored in saidfirst information storage layer being readout by said first laser beam;a second information storage layer which reflects said second laser beamtransmitted through said first information storage layer, theinformation stored in said second information storage layer beingread-out by said second laser beam; and a light incidence surface ontoand through which first and second laser beams are irradiated forread-out of the information from said first and second informationstorage layers, respectively, said light incidence surface being formedat one side of said optical disc, wherein, said first informationstorage layer and said second information storage layer are provided soas to be separated for separating the first and second laser beamsreflected by said first information storage layer and said secondinformation storage layer, and the length between said light incidencesurface to said second information storage layer is substantially equalto the thickness of said optical disc.
 2. An optical disc according toclaim 1 , wherein the length between said light incidence surface andsaid second information storage layer is substantially equal to 1.2 mmand the length between said light incidence surface and said firstinformation storage layer is substantially equal to 0.6 mm.
 3. Anoptical disc according to claim 1 , wherein said optical disc furthercomprises a spacer layer made by a transparent material and arrangedbetween said first information storage layer and said second informationstorage layer.
 4. An optical disc according to claim 1 , wherein thereflectivity of said first information storage layer to said first laserbeam at 635+20 nm is greater than equal to 20% and the reflectivity ofsaid second information storage layer to said second laser beam at780-830 nm is greater than equal 70%.
 5. An optical disc according toclaim 1 , wherein said optical disc further comprises a first and secondsubstrate, said light incidence surface being one surface of said firstsubstrate, and the other surface of said first substrate adjacent onsaid first information storage layer, one surface of said secondsubstrate being adjacent said second information storage layer, thethickness of said first and second substrate being substantially equalto 0.6 mm.
 6. An optical disc according to claim 5 , wherein said firstsubstrate and said second substrate are bonded together.
 7. An opticaldisc according to claim 5 , the other surface of said second substrateis bonded to another surface of said first substrate via said firstinformation storage layer.
 8. An optical disc comprising; a firstinformation storage layer made of a semi-transparent material and forreflecting a first laser beam but transmitting a second laser beam, thewavelength of said second laser beam being shorter than the wavelengthof said first laser beam, the information stored in said firstinformation storage layer being readout by said first laser beam; asecond information storage layer which reflects said second laser beamtransmitted through said first information storage layer, theinformation being in a standard compact disk format stored in saidsecond information storage layer being read-out by said second laserbeam; and a light incidence surface onto and through which first andsecond laser beams are irradiated for read-out of the information fromsaid first and second information storage layers, respectively, saidlight incidence surface being formed at one side of said optical disc,wherein said first information storage layer and said second informationstorage layer are provided to be separated for separating the reflectedlaser beam for said first laser beam reflected by said first informationstorage layer and the reflected laser beam of said second laser beamreflected by said second information storage layer, and the lengthbetween said light incidence surface to said second information storagelayer is substantially equal to the thickness of said optical disc. 9.An optical disc according to claim 8 , wherein the length between saidlight incidence surface and said second information storage surface issubstantially equal to 1.2 mm and the length between said lightincidence surface and said first information storage layer issubstantially equal to 0.6 mm.
 10. An optical disc according to claim 8, said optical disc further comprising a spacer layer made by atransparent material and arranged between said first information storagelayer and said second information storage layer.
 11. An optical discaccording to claim 10 , the thickness of said spacer layer is greaterthan equal to 30 mm.
 12. An optical disc according to claim 8 , whereinthe reflectivity of said first information storage layer to said firstlaser beam at 635+20 nm is greater than equal to 20% and thereflectivity of said second information storage layer to said secondlaser beam at 780-830 nm is greater than equal 70%.
 13. An optical discaccording to claim 8 , wherein the reflectivity of said secondinformation storage layer to said first laser beam is lower than thereflectivity of said second information storage layer to said secondlaser beam.
 14. An optical disc according to claim 8 , wherein saidoptical disc further comprises a first substrate and a second substrate,said light incidence surface being one surface of said first substrate,the other surface of said first substrate being adjacent said firstinformation storage layer, one surface of said second substrate beingadjacent said second information storage layer, the thickness of saidfirst and second substrate being substantially equal to 0.6 mmm.
 15. Anoptical disc according to claim 14 , wherein said first substrate andsaid second substrate are bonded together.
 16. An optical disc accordingto claim 14 , the other surface of said second substrate is bonded toanother surface of said first substrate via said first informationstorage layer.
 17. An optical disc read-out method for an optical disccomprising, (1) a first information storage layer which reflects a firstlaser beam but is transmissive with respect to a second laser beam, thewavelength of said second laser beam being shorter than the wavelengthof said first laser beam; (2) a second information storage layer whichreflects said second laser beam transmitted through said firstinformation storage layer; and (3) a light incidence surface onto andthrough which first and second laser beams are irradiated for read-outof the information from said first and second information storagelayers, respectively, said light incidence surface being formed at oneside of said optical disc, said first information storage layer and saidsecond information storage layer provided so as to be separated forseparating the reflected laser beam reflected by said first informationstorage layer and the reflected laser beam reflected by said secondinformation storage layer, a length between said light incidence surfaceto said second information storage layer being substantially equal tothe thickness of said optical disc, said read-out method comprising thesteps of: selecting between said first and second information storagelayer from which to read information stored therein; irradiating theselected information storage layer with the first laser beam if thefirst infrared layer was selected or the second laser beam if the secondlaser beam is selected, reading out the stored information for theselected information storage layer.
 18. An optical disc read-out methodaccording to claim 17 , wherein the wavelength of said first laser is635+20 nm.
 19. An optical disc read-out method according to claim 17 ,wherein information in a compact disk format is stored in said secondinformation storage layer.